There exist, particularly along the Coast of the United States bordering the Gulf of Mexico, geo-pressured aquifers or water reservoirs containing water and natural gas, which exist at pressures substantially higher than hydrostatic pressure. The geo-pressured aquifers, when in communication with a well bore, will flow water to the surface of the ground in artesian fashion.
These geo-pressured aquifers are known to contain natural gas dissolved in the water and while the total quantity of such gas reserves stored in these aquifers is not known with precision, various estimates range upward to 4,500 trillion cubic feet of gas. Natural gas may be present in geo-pressured aquifer formations in the form of (1) gas dissolved in the water, and also in the form of (2) a free-gas phase dispersed with water within the sand pores. An additional form of natural gas may exist in depleted and non-commercial geo-pressured gas reservoirs where (3) a free-gas phase is present separate from water. It is estimated that the recovery of water by artesian flow from such a bounded aquifer is in the range of 3% of the water contained in the bounded aquifer. If gas exists in such a bounded aquifer in solution with the water (form 1), the recovery of gas has been estimated by others to be the same 3% range which is not sufficient to make drilling of wells commercially successful. Geo-pressured aquifers having a dispersed free gas phase (form 2) generally contain a larger quantity of gas than aquifers containing only solution gas, but the amount of free gas in such aquifers cannot be conventionally recovered because of the co-production of large quantities of water. Natural gas production from commercial geo-pressured gas reservoirs frequently ceases because of large quantities of water production interfering with gas recovery. In addition, many geo-pressured reservoirs where gas production has been discontinued due to water interference with producing wells contain deposits of gas-phase gas (form 3) unrecovered but in pressure communication with existing wells.
The present invention is directed to producing water from wells completed in geo-pressured aquifers so as to maximize the recovery of natural gas from these formations. The conventional method of producing hydrocarbon fluids from oil or gas wells is to limit the rate of flow such that the formation pressure in the vicinity of the well is not reduced drastically so that water is not drawn into the well bore in appreciable quantities. On the other hand, the present invention is directed to producing water from the wells in an area of the reservoir having a dispersed free-gas saturation (form 2) at a high production rate so as to reduce the formation pressure significantly and preferably as quickly as possible whereby a significant fraction of the gas dissolved in the water contained in the aquifer is disdissolved (released from solution in the water). The natural gas, released from the water that remains in the aquifer, can then flow to the well independent of the flow of water and be recovered when a small free-gas phase is dispersed within the aquifer, because natural gas flows through the formation more easily than water due to the extreme contrast in viscosity of the two materials and due to the favorable gas/water permeability ratio.
The present invention is directed to a method which may produce as much as 14% of the gas in a geo-pressured aquifer as compared to present estimates of a recovery of only 3% by conventional methods.